Magnetic bearings are well known in the art. Magnetic bearings are commonly utilized for supporting a rotatable or oscillatory mass. An active magnetic bearing is a device which supports the mass in an actively controlled magnetic field. Typically the mass is supported by a plurality of radial magnetic bearings, and one or more thrust magnetic bearings. In most cases, these types of magnetic bearings use opposing attractive magnetic fields to support and control the mass. Thus, support along a given axis is obtained by balancing the pull of two opposing magnetic fields established by the bearings. Unfortunately, systems utilizing such bearings are inherently unstable. Closed loop control, which is based on the position of the supported mass, is required to provide stability. Toward this end, current state of the art active magnetic bearings include separate position sensing systems for measuring the position of the supported mass. This position information is then processed by a control system which regulates the current in the electromagnetic coils of actuators within the bearings. The current provides the magnetic flux for the gaps between the actuators and the supported mass. The magnetic flux provides the force required to control the position of the mass.
Several technologies have been used to provide position sensing information in association with magnetic bearings. Such technologies include the use of eddy current sensors, optical sensors, capacitive sensors, and reluctance sensors as position sensor elements. Each of these technologies has in common the fact that a sensor element is required separate from the magnetic bearing actuator. Such position sensor elements are typically mounted outside of an area occupied by the magnetic bearing itself for several reasons. These reasons can include restrictions on the physical space available within the bearing actuator, non-compatibility of the position sensing system to actuator environment, and electromagnetic interference between the bearing actuator and the position sensor.
FIG. 1 shows a radial cross sectional view of a typical radial magnetic bearing 20 containing an inductive type sensor. More specifically, the bearing 20 includes a housing 22 which surrounds an actuator element 24. The actuator element 24 includes an electromagnetic coil 26, and an end plate 28 is disposed adjacent the actuator element 24. The supported mass 30 includes a rotor carrier 32 on which rotor laminations 34 are formed. A rotor end plate 36 is disposed adjacent the laminations 34 as shown.
An inductive sensor element 40 is positioned within the housing 22 separate and axially offset from the actuator element 24. Specifically, a spacer element 41 is positioned between the sensor element 40 and the actuator element 24. The supported mass 30 also includes a sensor rotor 44 positioned proximate the sensor element 40, with a spacer 46 between the sensor rotor 44 and the rotor laminations 34. In addition to being physically separate from the actuator element 24, the sensor 40 requires its own driving coils 42 which require separate driving electronics. Also, in some instances (such as canned applications discussed below) it is desirable to isolate electrically the sensor 40 from the actuator 24.
FIG. 2 shows a similar radial cross sectional view of a radial bearing, but with a sensor configuration typical of a capacitive, optical, or eddy current type position sensor element 50. Note that the sensor element 50 consists of a separate probe that is installed so as to be axially offset from the bearing actuator element 24. These types of sensor elements 50 also require separate driver electronics.
The physical separation between the position sensor elements and the magnetic bearing itself introduces an instability mechanism which is commonly referred to as a "non-co-location problem". This problem manifests itself when magnetic bearings are applied to flexible shaft systems in which a shaft node or stationary point occurs between the bearing centerline and the measurement system centerline location. This axial position offset of the bearing and the sensor element(s) forces the bearing to respond to shaft vibrations in an inconsistent (out of phase), and de-stabilizing manner, with potentially catastrophic results.
Furthermore, in many applications it is desirable to isolate the magnetic bearing actuator from a surrounding hostile environment, where hostile could be defined as a hot, high pressure, caustic fluid, or other un-hospitable substances. One method of doing this, which is typical within the industry, is to introduce a thin non-magnetic protective material barrier, such as stainless steel, between the actuator and the environment. This barrier, which is referred to as a "can" within the industry, environmentally protects the magnetic actuator, while providing the magnetic flux with an unimpeded path to the suspended mass. Thus, the mass can be completely surrounded in a hostile environment without deleterious effects on the magnetic bearing system.
Unfortunately, existing position measurement systems must have an unobstructed view of, or access to, the supported mass. This means that those sensors, such as depicted in the prior art of FIG. 2, can not be protected by a "can" as is the bearing actuator. This then requires developing a sensing system that can be environmentally isolated to the same extent as the bearings.
It is therefore imperative for successful operation of any magnetic bearing system, especially with operation in hostile environs or with the potential for a non-co-location problem, to develop a position sensing system that is integrated into the magnetic bearing envelope. Moreover, it is desirable that such a position sensing system utilize one or more sensors which do not adversely affect or degrade the performance of the magnetic bearing.